The advantage of having a testcase management tool in trac is that it removes the need to have yet another system that manages testcases...with all the usual problems of maintaing user account, system upgrades, licensing issues. The other advantage is that it adds more transparency to the testing process as the tool and any testcases are right there for anyone to see.

Test cases are stored in a very simple XML format. And you can specify collection of test cases (for example a smoke test) by specifying which test cases belong to a test template. This information is also specified in an xml file.